Postbox supports a rich code editor within the Compose window, powered by CodeMirror.
Within the Compose window, click the code button </> to switch into Code View.
Here you can take advantage of its many features, including these:
Customizable Themes for Code View
To change your default Code View theme, go to Postbox (macOS)/Tools (Windows) > Preferences/Options > Composition > Advanced.
To change the Code View theme just for the current message, go to View > Theme.
Auto-Completion of HTML Tags
Hold down your control/alt-space key as you type to auto-complete HTML tags based on a built-in library of them.